In the past I've made rainbow cupcakes as well as a single layer rainbow cake. Last weekend I decided to make a 2 layer rainbow cake, with fresh cream icing in between the layers as well as covering the cake.
I used a very simple cake recipe that I always use for cakes, which basically works in ratio's so you can make a small or very large cake depending on if you double/triple the ingredients. Basically it's one egg for every 2 ounces of flour, sugar and butter. I used a 10 ounce recipe, which needed 10 ounces each of supreme sponge flour, caster sugar and softened butter/margerine, 5 eggs, and a teaspoon on vanilla extract. I use supreme sponge flour as I think my cakes turn out better, but you could also use self-raising flour (which I use when I can't find the supreme sponge flour). If you have a cake recipe that you already like, then simply use that and just divide the mixture and add colour =)
For the rainbow colours, I used Wilton food colouring gel, which I bought from a local party supply shop (but you can also purchase them online). The gel food colouring is very important as it means that the texture and taste of the cake mixture is not changing, whereas if you were to add liquid food colouring, you would end up with a very wet cake that wouldn't taste nice at all! Also, with the gel colours, you only need to use a tiny amount (about a quarter of teaspoon) to get brightly coloured cakes!
To layer the colours, I fill freezer bags with the coloured mixture and then pipe the colours on top of one another. When I first made the cakes, I did use a spoon and try to level out each layer, but piping the layers is a lot quicker and the colours are less likely to mix together.
I didn't manage to get step-by-step pictures, but here are the ones that I did manage to take =)
Cream butter/margerine and sugar together
Mix in eggs, then sift flour in a little at a time and mix until combined. Also add the vanilla extract at this stage.
Separate mixture into roughly five equal portions. Add the gel food colouring and mix well.
Pipe the layers of colour in the cake pans. Use the back of a spoon/palette knife to smooth down each layer. Extra cake mixture used for individual cakes =)
Bake in a pre-heated oven until cakes are baked through. Leave the cake to cool and then stack and decorate =)
Will you be trying to make rainbow cake any time soon?